Place chuck roast in a large slow cooker. Season with oregano, chili powder, cumin, salt and pepper. Add enchilada sauce, chicken bouillon cubes, chicken broth, tomatoes, garlic cloves, onion and dried Chiles. Cover and cook on low for at least 8 hours.
Remove meat from slow cooker and let cool for 10 minutes. Shred with two forks and set aside.
Transfer tomatoes, onion, garlic and dried Chiles from the slow cooker to a blender. Add 1 cup of the sauce leftover in the slow cooker. Blend until smooth.
Make the quesadillas: spread about ¼ cup queso quesadilla cheese over ½ a tortillas. Top with a spoonful of meat, a spoonful of the blended sauce, a sprinkle of diced white onion and cilantro, and a sprinkle of cheese on top. Fold tortilla over and grill in a hot pan until browned on both sides and cheese is melted, about 4-5 minutes per side.
Serve quesadillas with blended sauce on the side.
